SOFT SWITCHING CIRCUITRY

Abstract: Switching circuitry for use with an autotransformer for the smooth transition of voltage output from a circuit of either d.c. or a.c., between a higher and a lower voltage. The circuit includes at least one, preferably two, variable resistors (13) comprising a first solid state switch (10), an inductor (11) and a resistor (12), in which the variable resistor (13) is connected to the conductance side of the switch and wherein the circuit is connected to the voltage ouput of an autotransformer. If the solid state switches used are FETs the variable resistor (13) is placed within a bridge rectifier (D2-D5). Transition time for movement of the voltage between two levels is approximately one second.

Description: TITLE: SOFT SWITCHING CIRCUITRY TECHNICAL FIELD The present invention relates to switching circuitry which permits the gradual, smooth progressive change up or down of the voltage output of a switching circuit. This prevents any sudden alteration in the voltage output of control circuitry for electrical and/or electronic equipment. BACKGROUND ART At present when an electrical current is used in one of two modes (for example, normal mains voltage and a lower than normal voltage) , the switching between one voltage and a second often needs to be done smoothly and with no noticeable interruption to current flow. The problem of smooth switching is particularly relevant when related to the economic use of fluorescent lights. On start up, for example, it is important that such lights or lighting systems be at full voltage. Afterwards however the voltage can be reduced to reduce power consumption. Prior art voltage controllers using a single autotransformer for each phase of a.c. power are known. Examples are shown in WO88/03353 (Econolight Ltd) for non-fluorescent lights; US Patent No 4219759 (Hirschfeld) for fluorescent lights on three phase power; US Patent No 4189664 (Hirschfeld) for fluorescent power on single phase power; and US Patent No 4513224 (Thomas) for fluorescent lights in which contacts are used to select the tappings of an autotransformer to reduce the voltage applied to the lighting load. In DE 3736324 voltage reduction is achieved by switching a choke in series with the lighting load. The above citations have the disadvantages that there is either a requirement of critical contactor timing, breakage of the circuit during switching and no provision for a smooth or gradual change from one voltage to another. With some electrical equipment (for example fluorescent lights) this produces a noticeable flicker in the light level, which is undesirable. For other electrical equipment sudden changes in voltage are undesirable as they tend to reduce the working life of the equipment. An object of the present invention is to provide a switching circuit which permits a ramp transition between a higher and a lower voltage level (ramping up and down) of a current, while the circuit is in operation,which circuits also overcome the disadvantages of the above prior art circuits. A further object of the invention is the provision of such circuitry for use with either a.c. or d.c. currents. DISCLOSURE OF INVENTION The present invention provides an electrical switching circuit for ramp transition of the voltage output of said circuit between a higher and a lower voltage (or vice versa) , said circuit including: at least one variable resistor comprising a first solid state switch, an inductor and a resistor; and one or more second solid state switches; wherein one said variable resistor is connected to the conductance side of one said second switch; and wherein said circuit is connected to the voltage output of an autotransformer. Preferably, each said first solid state switch is a field effect transistor ("FET") . Preferably, if said circuit is to be used in conjunction with a.c. current, the circuit includes at least one diode bridge rectifier of known type. The bridge rectifier is not necessary if the circuit includes two systems of switching circuitry in back to back configuration. However more complex control circuitry than would otherwise be used is then required. Preferably, two variable resistors are incorporated in said circuit. Each said second solid state switch may be of any known type, for example, a TRIAC switch. Preferably also a capacitor is placed in parallel with the or each variable resistor to increasing the smoothing ability of said circuit. Where the a.c. circuit includes three phase power, a circuit as described above and an autotransformer can be used on each phase. BRIEF DESCRIPTION OF THE INVENTION By way of example only, a preferred embodiment of the present invention is described in detail with reference to use of the invention with the variation in the voltage output of an autotransformer and with reference to the accompanying drawings, in which:- Fig. 1 is a circuit according to a first preferred embodiment of the present invention; and Fig. 2 is a circuit according to a second preferred embodiment of the present invention. MODE FOR CARRYING OUT THE INVENTION Referring to Fig. 1, a first preferred embodiment of the present invention is thereshown. A voltage divider network maintains the secondary winding of the autotransformer (not shown) between the high and the low output modes. The circuit of Fig. 1 includes a FET 10, an inductor 11 and a resistor 12, which in series form a variable resistor 13. A capacitor 14 is arrayed in parallel with the variable resistor 13. Four diodes (D2-D5) form a bridge rectifier for a.c. current. The above circuit of Fig. 1 works as follows: the FET 10 is turned on, and the current through the inductor 11 and resistor 12 slowly builds up. If the FET 10 is then turned off the current will continue to flow through the inductor 11 and resistor 12 via a diode Dl. By adjusting the duty cycle of the FET 10 the current through the resistor 12 can be controlled. This variable current creates the same effect as a variable resistor. The capacitor 14 and resistor 12 also assist in damping out any harmonics generated within the circuit. The FET 10 can be switched on and off at a high frequency with the current smoothed by the operation of the inductor 11 and capacitor 14. A FET 10 is used in preference to a TRIAC switch (or other solid state switch) as it can be switched on and off at much higher frequencies. However other types of switches can be used, if so desired. Other types of solid state switches may also be used, for example insulated gate bipolar transistors (IGBTs) . As the FET 10 can only switch d.c. current, the bridge rectifier is required. Referring to Fig. 2 a second preferred embodiment of the present invention is thereshown. The circuit contains two variable resistors (as described above) 23, 33 with FETS 20, 30 (and possible alternatives, as described above), resistors 22, 32 and inductors 21, 31, respectively) . Two bridge rectifiers with four diodes (D2-D5) , as described above, are used in connection with each variable resistor (23, 33). Capacitors 24, 34 are also used in the manner described above with reference to capacitor 14. At point A the current must be controlled, it cannot float. The current must also be able to flow in both directions. For switching the voltage output of the autotransformer 40 from a higher to a lower level, the circuit is operated as follows: variable resistor 23 is switched to the shorted mode (FET 20 is closed and FET 30 is open) . The TRIAC switch 25 is switched off. Variable resistor 23 ramps down to an off position. At the same time variable resistor 33 is ramped on and the TRIAC switch 35 is operated, shorting A to Ν. Once the voltage output has been altered, both variable resistors 23, 33 can be turned off. This leaves the voltage output at the lower voltage level. The TRIAC switches 25, 35 then carry the permanent condition of the circuitry. If it is desired to raise the voltage to the higher level, the above described operations are reversed. If so desired, the TRIAC switches 25, 35 may each or both be replaced with FETs (not shown) . These must be placed within the bridge rectifier for each variable resistor (23, 33) and connected between the positive and negative terminals of the respective bridge rectifier. If so desired, the point E part way along the autotransformer can be moved to the end of the transformer, at point F. If so desired, the TRIAC switches described above can be replaced with any other suitable switch. The inductors 14, 24, 34 may be any appropriate type (high impedance for a.c. current, low impedance for d.c. current). The size of resistors (12, 22, 32) used will depend on the power and voltage levels. For example, with moderate kilowatt power levels and mains voltage of 240 volts, 100 ohm resistors are appropriate. If so desired, any other resistor capable of handling the power and voltage in the circuit may be used. In practice it has been found that a suitable transition time for the movement of the voltage from one level to the other (in either direction) is approximately one second. If so desired, this time can be shorter or longer, depending on the capabilities of the resistors used. In both preferred embodiments the order of the positioning of the inductors (11, 21, 31) and resistors (12, 22, 32) can be reversed. Other variants of the circuit arrangement are possible. Equally, the circuit can be configured in manners other than those shown in Figs. 1 and 2, and still obtain the same functional output from the circuit. The first preferred embodiment of the present invention can be used for control of a d.c. motor. The second preferred embodiment of the present invention can be used to control any electrical or electronic devices using a.c. current which, once switched on, can also operate at a lower voltage. An example of such devices is lights, especially fluorescent lights.

Claims: CLAIMS : 1. An electrical switching circuit for smooth transition of the voltage output of said circuit between a higher and a lower voltage (or vice versa) , said circuit including: at least one variable resistor comprising a first solid state switch, an inductor and a resistor; and one or more second solid state switches; wherein one said variable resistor is connected to the conductance side of one said second switch; and wherein said circuit is connected to the voltage output of an autotransformer. 2. An electrical switching circuit as claimed in claim l wherein each said first solid state switch is a field effect transistor ("FET") . 3. An electrical switching circuit as claimed in claim 2 wherein said circuit includes at least one diode bridge rectifier when the circuit is used in conjunction with a.c. current. 4. An electrical switching circuit as claimed in either claim 1 or claim 2 wherein said circuit includes two variable resistors. 5. An electrical switching circuit as claimed in any one of the preceding claims wherein said circuit includes one or more capacitors, one capacitor being placed in parallel with each variable resistor. 6. An electrical switching circuit as claimed in any one of the preceding claims wherein said second switches are selected from the group consisting of: TRIAC switches, FETs, and insulated gate bipolar transistors. 7. An electrical switching circuit as claimed in any one of the preceding claims wherein the transition time for the movement of voltage output from the circuit from one level to another is approximately one second.

Inventors: PEACH DAVID LESLIE LLOYD, PEACH, DAVID, LESLIE, LLOYD
Applicants: PEACH DAVID LESLIE LLOYD, SOUTHPOWER LTD, PEACH, DAVID, LESLIE, LLOYD, SOUTHPOWER LIMITED
Application year: 1994
Main classification: H05B41
IPC codes: G05F1, H02M5, H02P13, H05B41